Positive selection of T cells.

In the past year, significant technical developments have provided the opportunity to investigate the more mechanistic features of positive selection. Major progress has been made in determining the structure and function of the early pre-T cell receptor, in defining cell types that mediate positive selection, and in analyzing the contribution of MHC and co-receptors to CD4/CD8 lineage commitment. The most revealing studies have been those addressing the role of peptides in thymic selection.